Work machines, such as agricultural tractors, are commonly used to pull ground-engaging work implements, e.g., plows, disks, cultivators, and the like through the earth to perform desired work functions. As the machines pull the implements, it is common for the characteristics of the earth to provide a varying degree of resistance to the implement as it works the ground. This resistance must be overcome by the machine to allow continuous movement along the terrain.
Quite often, the resistance provided by the ground-engaging implement must be compensated for to allow the machine to traverse the terrain as desired. For example, if the resistance increases beyond certain values, the work machine may begin to stall, or the tires or tracks of the machine may slip on the ground. Typically, the excess resistance is compensated for by raising the implement until the resistance is within acceptable limits.
As an example of an attempt to compensate for ground resistance caused by a ground-engaging implement, in U.S. Pat. No. 4,518,044, Wiegardt et al. (Wiegardt) monitors various parameters on a tractor such as wheel slip, engine speed, draft force, and implement position, and responsively raises or lowers the implement to balance implement resistance with a desired implement depth. Sensors on the draft links or drawbar of a three point hitch are used to sense draft load. Although Wiegardt may control the resistance of the implement, a major disadvantage is that the implement cannot be maintained at the desired working depth, thus sacrificing work efficiency and accuracy. In addition, some of the monitored parameters, e.g., wheel slip and engine speed, indicate the need to take corrective action after a problem has begun. For example, the detection of wheel slip indicates that the implement resistance has already exceeded a desired tolerance, and thus the increased resistance has already caused problems.
Attempts have been made to control implement resistance with greater precision and responsiveness. For example, it is known that control of the depth, and therefore the resistance, of a ground-engaging implement can be used to shift the load point on the tractor itself; that is, the load on the tractor may be shifted along the longitudinal length of the tractor to a desired point. It may be desired to maintain a load on the tractor during normal operating conditions that is balanced at about the center portion of the tractor, and shift the load point to the drive engaging wheels, e.g., the rear wheels, during periods of excessive implement resistance when wheel slippage is likely. The additional load placed on the drive wheels places more weight on the wheels and reduces wheel slip.
In U.S. Pat. No. 3,825,072, Collins discloses a system which senses draft load on the upper link of a three point hitch, and responsively raises or lowers the implement to distribute the weight, i.e., load, on a tractor to provide increased traction when needed. Once again, however, the implement is not maintained at a constant desired depth, so productivity and accuracy is sacrificed for the sake of increased traction.
It is desired to provide a method for shifting the load on a work machine to a desired location without changing the working depth of the implement being pulled by the machine.
The present invention is directed to overcoming one or more of the problems as set forth above.